Refrigeration apparatus employing liquified gas

ABSTRACT

Refrigeration apparatus is provided which is motivated by the vaporization of liquefied nitrogen or the like. In particular, nitrogen vapor is introduced into a primary heat exchange coil at a high vapor pressure, conducted from the primary coil at a lower pressure through a pneumatic motor, and thereafter through a secondary heat exchange coil from which it is vented. The pressure drops across the two coils will chill the coils, and blowers are driven by the motor to circulate the atmosphere to be cooled over the coils. Defrosting means is included for automatically re-routing the liquefied nitrogen intake stream through a heater whenever ice accumulates on the coils in an excessive amount or ambient temperature drops below the temperature sought to be maintained.

Elted States Patent Gement, Jr.

1 Sept. H, 1973 REFRIGERATION APPARATUS EMPLOYING LIQUIFIED GAS Inventor: Paul M. Gement, Jr., PO. Box

10593, 4 Knox Rd., New Orleans, La. 70121 Filed: July 9, 1971 Appl. No.: l6l,25 8

References Cited UNITED STATESYPATENTS 3/1968 Boese 62/514 Primary Examiner-Meyer Perlin Att0rneyDonald l-l. Fidler et al.

[57] ABSTRACT Refrigeration apparatus is provided which is motivated by the vaporization of liquefied nitrogen or the like. In particular, nitrogen vapor is introduced into a primary heat exchange coil at a high vapor pressure, conducted from the primary coil at a lower pressure through a pneumatic motor, and thereafter through a secondary heat exchange coil from which it is vented. The pressure drops across the two coils will chill the coils, and blowers are driven by the motor to circulate the atmosphere to be cooled over the coils. Defrosting means is included for automatically re-routing the liquefied nitrogen intake stream through a heater whenever ice accumulates on the coils in an excessive amount or ambient temperature drops below the temperature sought to be maintained.

[6 Claims, 4 Drawing Figures TO DRAIN PAN DEFROST OUTS/DE TRAILER PATENTED UI m1:

' suenaur4 LN2 FEED SPRAY coou/ve PATENTEI] SEN 1 I975 SHEET a or 4 FROM CONTROL C/RCU/T REFRIGERATION APPARATUS EMPLOYING LIQUIFIED GAS BACKGROUND OF INVENTION This invention relates to apparatus for establishing and regulating the temperature of a defined space and, more particularly, relates to improved refrigeration apparatus for truck trailers and the like.

It is well known to employ trucks and other types of motor vehicles to transport foodstuffs and other perishables, and it is also well known to refrigerate the storage interiors of such vehicles for the purpose of prolonging the useful life of such foodstuffaFurthermore, it is well known to employ large van-like trailers for this purpose, which may be pulled over the highway by a truck or tractor during part of the journey, and which may also be detached from the truck and loaded aboard a railroad car for another part of the journey.

It will be apparent that a typical journey for conveyances of this type may often continue for many hours. What is not generally known, however, is that a trailer is often required to stand for many hours before it is either forwarded on to another destination or before it can be unloaded. Although these trailers are quite well built and well insulated, of course, it will thus be apparent that they must be continually refrigerated throughout the entire period during which perishables are stored therein.

Trailer equipment of this type was originally refrigerated by having their walls lined with ice or frozen carbon dioxide. However, most trailers are now provided with some form of refrigeration system which is adapted to be energized either by power from the truck or prime mover which tows the trailer, or from a separate power supply which is mounted in .or on the trailer itself. I

Trailers of this type have long been used on a widespread commercial basis, and many improvements have been made in their refrigeration systems, as well as in the trailers themselves. Notwithstanding that many different kinds of systems have been built and used, however, there are disadvantages with each of the refrigeration systemsnow in use. Moreover, most if not all of the most serious of these disadvantages may be traced to the means by which the systems are powered.

For example, some systems employ compressors which are driven by electric motors, and these motors in turn are supplied by the generator or alternator which is in the prime mover. This is satisfactory as long as the prime mover and trailer are traveling on the highway. if it becomes necessary to hold the trailer in abeyance for any extended period, however, it then becomes necessary for the driver to remain in constant attendance to make certain that the truck motor neither stalls nor overheats.

It is often the practice for two or more different freight companies to assume the task of moving the trailer for difi'erent portions or legs of the journey, and

else to be able to quickly adapt almost any kind of prime mover to any kind of trailer.

A further disadvantage to motivating a trailer refrigeration system by power derived from the prime mover is that the trailer must be separated from its prime mover whenever the trailer is to be forwarded by railroad. Thus, the trailer refrigeration system will be without power s long as the trailer-is aboard the train unless special precautions are taken which are usually neither convenient nor even available.

In many instances, the trailer is provided with an auxiliary power supply which is employed whenever the power supply in the prime mover is disconnected or deenergized. Accordingly, batteries and even auxiliary gasoline-powered engines have been added to the trailer to serve as the auxiliary power supply to which resort may be had under such circumstances.

Recently, a new type of refrigeration system has been developed which is fairly depicted in US. Pat. No. 3,406,533, and which is motivated by the evaporation of a supply of liquid nitrogen or the like. In particular, a system of coils are arranged appropriately within the interior of the trailer (or other space to be refrigerated) and are coupled to the outlet of a bottle" of liquid nitrogen. Accordingly, provision is made for transformation of the nitrogen from a liquid to a gas, and further that such transformation occur within the coils, whereby air flowing over the coils will be appropriately chilled. In an open system of this type, the resulting nitrogen gas is not reliquefied as in sealed systems, but is either discharged within the trailer for accelerated cool-down or else the nitrogen gas is merely vented out of the trailer. I

It will be apparent from the foregoing that the main source of energy for a system of this type is in the liquefied nitrogen which is stored in the bottle. It will also be apparent that such a power supply needs no attendance, and since it is preferably located in-the trailer it is completely independent of the type and even the existence of the prime mover. In addition, such a power supply is almost percent reliable and requires al most no maintenance or repair.

Notwithstanding the aforementioned advantages which must be credited to such a refrigeration system, it is nevertheless a fact that such a system is also subject to certain disadvantages which severely limit the usefulness of such a system. For example, it will be noted that in the system depicted in the aforesaid US. Pat. No. 3,406,533, the fans are included for circulating the air in the trailer across the cooling coils of the system, and these fans are driven by electric motors. It is a fact that all coil-type systems employ electrically-actuated control devices. However, there is no disputing that electrical motors constitute a much heavier drain on the batteries which must be included, and that the addition of batteries to the system re-introduces a critical element of uncertainty and undependability.

These and other disadvantages of the prior art are overcome by the present invention, and novel methods and apparatus are provided herein for refrigerating the interior of a trailer and the like. More particularly, improved trailer refrigeration methods and apparatus are herewith provided whereby the temperature of the interior of a truck or trailer or the like may be established and thereafter maintained at a preselected value independently of fluctuations in the temperature outside of such trailer.

SUMMARY OF INVENTION In an especially suitable embodiment of the present invention, a cooling coil assembly is provided which includes a primary cooling coil, a separate secondard cooling coil, and a turbine or other pneumatic motor interconnected therebetween. More particularly, the discharge end of the primary coil is coupled to the intake port of the pneumatic motor, and the intake port of the secondary coil is coupled to the discharge port of the motor. The primary coil is preferably coupled through suitable shutoff equipment to a tank or bottle of liquid nitrogen, whereby liquid nitrogen may flow to and perhaps into the primary coil, vaporize within the primary coil to drive the pneumatic motor, and thereafter traverse the secondary coil before being vented to ambient atmosphere or being discharged into the space to be refrigerated.

The purpose of both the primary and secondary cooling coils is, of course, to remove heat from the air or other atmosphere in the space sought to be refrigerated. Accordingly, fans or other conventional circulating means are preferably included and, in the present invention, these fans are preferably motivated by the pneumatic motor instead of by an electric motor as in the case of many prior art systems.

it will be noted that although there is a substantial pressure drop between the intake and discharge ports of the primary coil, the pneumatic motor nevertheless acts as a restriction on the primary cooling coil, whereby pressure at the discharge end of the primary cooling coil will nevertheless be substantially above the pressure either inside or outside of the trailer. (As may be expected,- trailers of this type are usually heavily insulated against heat loss, but they are rarely if ever pressurized. Accordingly, atmospheric pressure in the trailer may be presumed to be the same as the atmospheric pressure outside of the trailer.) Thus, the pressure drop which will occur across the pneumatic motor will be much greater than that required to drive the motor, if the discharge end of the motor is vented to atmosphere, and the heat transfer capability of this excess pressure drop will be wasted insofar as the purposes of the invention are concerned.

it will thus be apparent to those with skill in this art that the secondary cooling coil contributes substantially to the effectiveness of the refrigeration system to which it is joined. in particular, it will be apparent that if the secondary cooling coil is properly joined to the discharge side of the pneumatic motor, the pressure differential across the motor will be substantially only that amount which is functionally related to the work performed by the motor. The discharge end of the secondary coil will necessarily be vented either inside or outside of the trailer. Accordingly, most of the pressure differential which is wasted insofar as heat exchange is concerned, when the discharge side of the pneumatic motor is vented to atmosphere, will now be developed across the secondary cooling coil where it can be utilized for refrigeration purposes.

it is recognized by those with experience in this art that the ultimate effectiveness of a refrigeration system of this type is a function of the differential between the temperature of the nitrogen at the instant of discharge into ambient atmosphere and the then temperature of the atmosphere within the space sought to be refrigerated. in a system which was constructed and operated according to the teachings presented herein, there was found to be substantially no measurable difference between the then temperature within the trailer and the temperature of the nitrogen vapor at zero psig, whereas with other systems of this type the temperature differential is sometimes as great as 40 F.

in another feature of the preferred embodiment of the present invention, which should be especially noted, means is preferably included for heating as well as cooling the nitrogen being supplied to the primary and secondary coils hereinbefore discussed. It is well known that, for various reasons, the cooling coils in a system of this type will frequently freeze over to the point that their heat transfer capabilities are substantially diminished. These systems are usually unattended during substantial time intervals, as hereinbefore stated, and thus a perishable cargo may be lost because of this type of failure of the refrigeration system.

This disadvantage of the prior art is also overcome with the present invention, since in this preferred embodiment of the present invention there is preferably included provision for automatically defrosting the cooling coils whenever freezing occurs or begins to occur. in particular, heating means is included with a control system which, when freezing occurs, detours the flow of liquid nitrogen to a heating coil or other unit, whereby the liquid nitrogen may be heated to a vapor of 250300 F. or more before being redirected to the cooling coils for purposes of thawing them. In a particularly useful aspect of this feature, the pneumatic motor is bypassed whenever the refrigeration system is transferred to the defrost condition or mode, whereby ice and water will not be blown off of the defrosting coils and onto the cargo.

, There is a further advantage which is available with such a system and which should be especially noted. As hereinbefore stated, trailers of this type are often carried long distances during a single trip or journey, and thus the weather (and thus the ambient temperature) to which the trailer is subjected may change radically before the trailer reaches its destination. In other words, the trailer may begin the journey while exposed to an ambient temperature of F., but it may subsequently move to a region where the ambient temperature has dropped to 30 F. or even less. if the trailer is refrigerated by a system of the prior art, and if the system has been pre-set to establish an interior temperature of 35 F. (for example), such a system will simply quit whenever the temperature in the trailer reaches the preselected temperature. If the trailer continues to be exposed long enough to an ambient temperature of 30 F., however, the temperature within the trailer will eventually decline to 30 F. Accordingly, this may result in damage to the cargo if the trailer happens to be loaded with a foodstufisuch as lettuce, which must be kept chilled but which cannot be frozen.

This disadvantage of the prior art is also overcome with the present invention. In particular, this system which embodies the present invention, further includes provision whereby the system shifts to the heating cycle whenever the temperature in the trailer drops below the temperature sought to be established and maintained.

As hereinbefore stated, any heating device may be employed which may be suitable under the circumstances. A heating device which has been found especially suitable for purposes of the present invention,

however, may be a conventional burner assembly which may use butane or some other liquefied petroleum as a fuel and which may also employ conventional regulating devices in conjunction with the control system hereinafter described.

These and certain other features and. advantages will become more apparent from the following detailed description of the present invention, wherein reference is made to the figures in the accompanying drawings.

IN THE DRAWINGS FIG. 1 is a simplified functional diagram which depicts the basic concept of one form of refrigeration system embodying the present invention, including the cooling coils, liquid nitrogen supply and the like.

FIG. 2 is a simplified functional diagram which depicts in basic form one embodiment of a control circuit which is suitable for use with the system which is depicted in FIG. 1.

FIG. 3 is also a simplified functional diagram depicting an alternative control circuit for regulating the system depicted in FIG. 1.

FIG. 4 is a simplified functional diagram for illustrating one means for controlling the blower system when the system in FIG. 1 is shifted to the defrost mode.

DETAILED DESCRIPTION Referring now to FIG. 1, there may be seen a simplified diagram of the more important functions and components of one type of refrigeration system which embodies the concept of the present invention. In particular, the system may be seen to include a suitable liquefied gas supply such as a tank or bottle 2 of liquid nitrogen coupled to the intake ports of a cooling control valve 3, a defrost control valve 5, and a so-called quick-chill" control valve 7. All three. of these compo nents are preferably pneumatic-actuated, as will hereinafter be apparent, although the quick-chill control valve 7 may conveniently be adapted to be manually positionable for purposes of emergency.

Even the most efficient and effective of coil-typerefrigeration systems will require a substantial period of time within which to reduce the temperature in a large trailer to a working level. Thus, there will always be occasions with any refrigeration system when it becomes necessary to cool down" the trailer in a much shorter period. Referring to FIG. 1, therefore, it will be seen that the quick-chill control valve 7 is coupled directly between the nitrogen bottle 2 and a plurality of jets or discharge nozzles 9 which are arranged appropriately in the trailer (not depicted). Accordingly, liquid nitrogen may be sprayed directly into the trailer, thereby refrigerating the interior of any such trailer within a very short time interval.

Referring again to FIG. 1, the cooling control valve 3 is preferably a normally-closed unit which is opened by an appropriate actuator 4. Similarly, valves 5, 7 and 20 are also normally-closed and are preferably opened only by actuators 6, 8 and 21. All of these actuators may be of any suitable conventional design, but for the reasons hereinbefore set forth, it is preferable that all valve actuators depicted in FIG. I be pneumatically energizable by pressure supplied by the nitrogen.

Referring again to FIG. 1, it will be seen that the dcpicted system preferablyv includes a primary cooling coil 14 having its intake end coupled to the cooling control valve 3 by way of a check valve 10, and having its discharge end coupled to the intake side of a turbine or other suitable pneumatic engine 16. The exit side of the engine 16 is preferably coupled to the intake side of a secondary cooling coil 15 having its discharge end coupled through a vent control valve 19 to either the inside of the trailer (see valve exit port 19A) or to the ambient atmosphere outside of the trailer (see valve port 198).

It will be apparent from FIG. 1 that the primary cooling coil 14 is preferably arranged in heat exchange relationship to the air or atmosphere surrounding the primary coil 14 and confined within the trailer (not depicted). In a coil 14 and confined within the trailer (not depicted). In a particularly satisfactory embodiment of the present invention, it was found that the discharge pressure at the intake port of the cooling control valve 3 is usually on the order of 30-40 psig and that the nitrogen which reaches the'intake side of the primary cooling coil 14 is substantially liquid in fonn; However, it was also found in this embodiment of the invention that the liquid nitrogen is substantially all converted to a vapor before reaching the exit side of the primary cooling coil 14, and that the pressure at the discharge side of the motor 16 is on the order of about half the pressure at the intake side of the primary cooling coil 14. Accordingly, it will be seen that the pneumatic motor 16 functions as a restrictor to support the pneumatic pressure in the primary cooling coil 14. 7 Referring again to the system depicted generally in FIG. 1, it will be noted that secondary cooling coil 15 is coupled between the discharge side of the pneumatic motor .16 and the intake port ofa vent valve 19 having one discharge port 19Alocated to communicate with the interior of the trailer (not depicted) or other space to be refrigerated and having another discharge port 19Bl0cated to communicate with ambient atmosphere or the like. In some instances, and especially in those instances-wherein the interior of the trailer has been quick-chilled, it may be desirable to maintain an inert atmosphere within the trailer. Accordingly, the vent control valve 19 may be positioned to open a flow path from the discharge end of the secondary cooling coil 15 through the discharge port 19A. In this position, a stream of nitrogen vapor will continually be discharged into the space to be cooled, and thus the lifesustaining air in the trailer will be replaced by an inert atmosphere which does not support the growth of bacteria.

The existence of an atmosphere within the trailer which cannot support life, however, is often a disadvantage since it is impractical for workmen to operate in the trailer under such circumstances. Accordingly, it is often preferred instead to vent the discharge end of the secondary cooling coil 15 into the interior of the trailer (not depicted). Thus, the vent control valve 19 is usually positioned to select the other discharge port 19B, whereby nitrogen vapor exiting from the secondary cooling coil 15 is merely vented to ambient atmosphere.

It will be noted that the pressure at the discharge end of the secondary cooling coil 15 is substantially at the same atmospheric pressure, irrespective of whether the vent control valve 19 has been positioned to select port 19A or port 198. Thus, the pressure drop which occurs across the secondary cooling coil 15 is nearly as great as that obtained across the primary cooling coil 14 and motor 16. Accordingly, the addition of the secondary cooling coil 15 contributes substantially to the effectiveness as well as the efficiency of the system depicted or suggested in FIG. 1.

It is conventional with all systems of this type to provide some means for circulating air across the particular chilling means which is being employed. Accordingly, the system embodying the present invention may also include a fan 17 of conventional design for circulating the air in the trailer across the primary cooling coil 17, and perhaps another separate fan 18 to perform the same function with respect to the secondary cooling coil 15. As previously explained, however, it is conventional to employ one or more electric motors to motivate such fans or circulation means, and thus the prior art systems are generally required to include a battery or some other similarly undesirable means to drive such circulation means. Referring now to the system depicted in FIG. 1, it will. be seen that this disadvantage of the prior art methods and apparatus has now been eliminated and that the pneumatic motor which is motivated by nitrogen flow between the two coils 1t and 15 is preferably coupled to rotate at least one of the two fans 17 and 18.

Referring again to FIG. 1, it will be noted that the defrost control valve is interconnected with a heat exchange coil 12 or other suitable means, and also with another check valve 11, to provide an alternative flow path between the nitrogen supply 2 and the intake side of the primary cooling coil 14. Further, it will also be noted that the heat exchanger coil 12 is arranged to be heated appropriately by the flames from a conventional burner 25 coupled to a suitable fuel supply 24 such as a bottle of butane or propane.

It is well known that it is frequently desirable to defrost the cooling coils in any type of refrigeration system. It will also be recognized by those with special experience in this technology, however, that there is a particular problem in achieving effective defrosting in a system of this type which is subject to large unpredictable changes in ambient temperature. Moreover, it will be apparent that the problem of effectively defrosting the system depicted in FlG. l is additionally complicated by the tandem arrangement of the primary and secondary cooling coils l4 and 15.

In particular, it may be seen that if the cooling control valve 3 is closed, and if the defrost control valve 5 is opened instead, liquid nitrogen will flow through the heat exchanger coil 12 to the intake side of the primary cooling coil 14. If the fuel control valve 23: is open, there may be a flow of fuel gas into the burner 24, and thus the liquid nitrogen entering the heat exchanger coil 12 may be vaporized and heated to whatever tenipcrature may be suitable for defrosting the primary coil 14.

When the defrost cycle has been completed, however, and the defrost control valve 5 is returned to its normally closed position, the solenoid 4 may again be energized to open the cooling control valve 3. Accordingly, liquid nitrogen will then be routed to the intake side of the primary cooling coil 14, and the depicted system will revert to the refrigeration cycle.

It will be noted that nitrogen which passes through the heat exchanger coil 12 may reach a temperature of 250-300 F. before entering the primary cooling coil M. This rise in temperature will be achieved in a relatively progressive manner, however, and thus hot nitrogen is not injected directly into a frozen coil 12 as might appear to be the case.

When the system is shifted from its defrost cycle to its refrigeration cycle, the situation is completely different. In other words, when the defrost control valve 5 is closed, and when the cooling control valve 3 is suddenly open, liquid nitrogen will tend to surge abruptly into the primary cooling coil 14. The defrosted coil 14 may itself have been heated to 200-250 F. by the hot gas out of the heat exchanger coil 12, however, and when liquefied nitrogen meets the hot coil 14 there tends to be a sudden expansion in the line away from the primary cooling coil 14 back toward the nitrogen bottle 2. The cooling control valve 3 is full of liquid nitrogen, and thus the main thrust of this expansion will be directed around through the heat exchanger coil 12 and against the closed defrost control valve 5. The usual result of such an impact is to blow out or otherwise rupture the seals in the defrost control valve 5.

Referring again to FIG. 1, it will be noted that the check valve 11 is arranged to pass fluid flow only from the heat exchanger coil 12 to the primary cooling coil 14. Accordingly, the principal function of this check valve 11 is to prevent backflow of either liquefied or vaporized gas into the defrost control valve 5 by way of the heat exchanger coil 12.

As hereinbefore stated, when the defrost control valve 5 is open and the cooling control valve 3 is closed, there is a similar tendency for fluid to explode between the cooling control valve 3 and the primary cooling coil 14. Thus, the other check valve 10 is included to prevent this expansion from damaging the seals in the cooling control valve 3.

Although the cooling coils l4 and 15 have been depicted in only a functional manner in FIG. 1, it should be noted that apparatus of this character is conventionally provided with auxiliary equipment such as a drain pan, drain line, etc., and that the drain pan (not depicted) also requires defrosting on occasion. Accordingly, it will be noted that the system depicted in FIG. 1 may conveniently include a drain defrost line 13 which is coupled to receive heated gas from the exit end of the heat exchanger coil 12, to direct such heated gas to the drain pan and drain line (not depicted), and also to any other portion of the interior of the trailer which may require defrosting. It will also be noted in this regard that the check valve 11 functions to prevent liquefied nitrogen from being directed into the drain pan and drain line when the cooling control valve 3 is opened.

Referring again to FIG. 1, it will be seen that the fuel control valve 23 which is coupled to the intake end of the burner 25 is preferably positioned according to the position of a pneumatically-actuated regulating valve 22 which, in turn, is controlled by the flow of nitrogen through the control valve 20. Accordingly, when the defrost cycle is to be initiated, valve 20 is preferably opened in conjunction with the opening of the defrost control valve 5, and also preferably in conjunction with the closing of the cooling control valve 3. Nitrogen pressure is then routed to the regulating valve 22, which is then appropriately positioned by such pressure to open the fuel control valve 23.

Referring back to FIG. 1, there may be seen a conventional thermostat or other suitable temperature indicator/regulator 27 having a temperature sensor 27A preferably disposed to measure the temperature of the air in the trailer which is to be blown over the coils I4 and 15, and further adapted to translate nitrogen pressure from the bottle 2 into a suitable pneumatic actuating signal 278. A suitable control circuit 26 is also preferably included which is at least substantially entirely pneumatic, as in the case of the embodiment more particularly depicted in FIG. 2, but which is preferably entirely pneumatic in character as in the example depicted in FIG. 3.

The regulator 27 preferably includes provision whereby a predetermined temperature is selected to be achieved and maintained in the cargo space within the trailer. Thereafter, the regulator 27 will continuously compare the actual temperature in the trailer (as indicated by the sensor 27A) with the predetermined temperature which has thus been selected, and, as long as the actual temperature in the trailer is greater than the temperature setting on the regulator 27, the regulator 27 will open a valve (not depicted) to route pneumatic pressure from the bottle 2 to the control circuit 26 in the form of a pneumatic signal 278.

The presence of the pneumatic signal 273 will, of course, cause the control circuit 26 to condition the system for the cooling mode. More particularly, the receipt of the pneumatic signal 278 causes the control circuit 26 to transmit a pneumatic command signal 26A to the pneumatic actuator 4, whereby the cooling control valve 3 is opened to route liquid nitrogen from the bottle 2 to the intake side of the primary cooling coil 14.

As hereinbefore stated, ice may form or accumulate over one or both of the two cooling coils l4 and 15. When this happens, the air pressure tends to increase in front of the primary cooling coil 14 because theair from the fan 17 tends to be blocked by the accumulating ice. As indicated in FIG. 1, the system preferably includes means for sensing this pressure change, such means preferably being a differential pressure switch or valve 28 having a pitot tube 28A disposed to respond to the air pressure from the fan I7, and further having.

an appropriate probe 285 disposed in the return air side of the two fans 17 and 18 and coupled to the other side of the diaphragm (not depicted) in the valve 28. If the back pressure which is caused by the ice (also not depicted) reaches a sufi'icient proportion, the switch or valve 28 will produce an appropriate actuating signal 28C.

When the signal 28C appears, the control circuit 26 will respond by first discontinuing the cooling command signal 26A to the pneumatic actuatord, whereupon the cooling control valve 3 will immediately revert to its normally closed position. Immediately thereafter, however (or simultaneously with the discontinuance of signal 26A), the control circuit 26 will produce another pneumatic command signal 268 which is applied to energize both the pneumatic actuator 6 for the defrost control valve and the pneumatic actuator 2! for the fuel safety control valve 20. Accordingly, the defrost control valve 6 will now open to route liquid nitrogen into the heat exchange coil 12, and the fuel safety control valve 20 will open the fuel control valve 23 to admit butane or other fuel gas from a similar pressure tank or bottle 24 to a conventional burner 25 for the purpose of heating the nitrogen in the heat exchange coil 12. Thus, hot nitrogen vapor will now flow through the check valve 11 to the drain line 113 and primary cooling coil I4, as hereinbefore suggested.

The timing circuit, which is composed of the accumulator 45 and orifice 44, will limit the defrost cycle to a predetermined time interval, i.e., 6-10 minutes, whereupon the relay 43 will revert to its original condition to discontinue the defrost signal 26B (thereby permitting control valves 5 and 20 to revert to their normally closed condition) and will instead generate the cooling command signal 26A, which reopens the cooling control valve 3 and which accordingly restores the system to its cooling mode.

It should be noted, of course, that the control circuit 26 will generate the cooling signal 26A only as long as it receives an actuating signal 27B from the regulator 27, and this is done only so long as the temperature measured by the sensor 27A is greater than the temperature setting of the regulator 27. Whenever the temperature being measured by the sensor 27A drops a sufficient amount below the setting of the regulator 27 (which is the temperature sought to be maintained in the trailer), the regulator 27 will discontinue the signal 273, and the control circuit 26 will, in turn, discontinue the cooling signal 26A and thereby close the cooling valve 3.

Referring now to FIG. 2, there may be seen a func-.

tional diagram which illustrates in greater detail one embodiment of the control circuit 26 referred to in FIG. 1 and which further illustrates the manner in which the control circuit 26 operates to shift the refrigeration system to either the cooling mode orthe defrosting (heating) mode, In this respect, it will be noted that although the form of the control circuit 26 is illustrated in FIG. 2 is not completely pneumatic in character, it is nevertheless especially suitable for the purposes of the present invention.

In particular, the apparatus depicted in FIG. 2 may be seen to include the bottle 2 of liquid nitrogen which is connected to deliver nitrogen vapor at 30-40 psig to the input side of a pressure reducing valve 34, to a toggle-type shutoff valve 33, to the input side of a threeway valve 40, a pressure reducing valve 42, and also to a pneumatically-actuated relay 43. The reducing valve 34 functions to reduce the incoming nitrogen vapor pressure to 17-20 psig and to pass the nitrogen on to the input side of the regulator 31. In this respect, sensor 31A corresponds functionally to the sensor 27A depicted in FIG. 1, and thus the temperature/pressure regulator 27 depicted in FIG. I may be functionally equated to the regulator 31, valve 32, and regulator 34 depicted in FIG. 2. Similarly, valves 4% and 38 and the reducing valve 42 and relay 43 are all component parts of any apparatus which is the functional equivalent of the control circuit 26 depicted in FIG. 2.

Referring again to the regulator or controller 31 in FIG. 2, it should be noted that the temperature sensor 31A for the regulator or controller 31 is preferably mounted in the return air supply (not depicted) of the system illustrated in FIG. 1. Thus, if the sensor 31A indicates that the temperature in the trailer is higher than that sought to be maintained, the controller 31 will route 17 psig pneumatic pressure to the bellows or pneumatic actuator 41 which controls the three-way valve 40. As indicated, valve 40 now changes condition to route 30-40 psig nitrogen pressure through valve 38 to the pneumatic actuator 51 which corresponds to the actuator 4 depicted in FIG. 1. Accordingly, the cooling control valve 3 depicted in FIG. I will now be opened, and the system will now be in the cooling mode.

A pilot tube 52 or other functional equivalent of the sensor 28A may conveniently be mounted in the trailer to respond to the discharge of whatever circulation system may be installed therein and may be conveniently interconnected with an appropriate pressure probe 53 to apply a corresponding pressure differential to the diaphragm (not depicted) of a pressure differential switch 49 of conventional design. As long as the flow rate through primary cooling coil 14 continues at an adequate level, the normally-open pressure differential switch 49 will be closed to maintain electrical power on the solenoid (not depicted) of the normally closed shutoff valve 46. The magnitude of electrical power required for this purpose is very small and may conveniently be derived from a conventional thermopile generator 48, or the like, which is conventionally positioned in the pilot flame 56 which ignites the main burner 25 in FIG. 1. Note that an indicator light 47 may also be conveniently interconnected with the thermopile generator 48 to furnish a continuous indication of the condition of the pilot flame 56. When the shutofivalve 46 is open, nitrogen pressure at about 1% psig will be routed from the pressure reducing valve 42 to energize the relay 43, and also to fill an accumulator 45 which is also a part of the functional' equivalent of the control circuit 26 in FIG. 1.

After the accumulator 45 has been filled, it will then discharge or bleed out through an orifice 44 of a predetermined size such as will determine the rate of such discharge. When the discharge is completed, the relay 43 will close. Until the accumulator 45 discharges, however, the relay 43 remains open to route30-40 psig pressure directly from the bottle 2 to the pneumatic actuator 39 which repositions the two-way valve 38. Accordingly, pressure is now disconnected from pneumatic actuator 51 (to close the cooling control valve 3), and is coupled instead to the pneumatic actuators 36 and 37. Pneumatic actuator 37 '(whieh corresponds functionally to actuator 6) willnow open the defrost control valve 5. Pneumatic actuator 36 (which corresponds to actuator 21) will also open the control valve to thereby route actuating pressure to the valve 22 whichcontrols the fuel shutoff valve 23.

Referring again to FIG. 2, it may be seen that the manually operated toggle valve 33 is preferably interconnected to bypass the entire system and, in lieu thereof, to route liquefied nitrogen at 30-40 psig vapor pressure directly to the pneumatic actuator 50 which is the functional equivalent of the actuator 8 depicted in FIG. 1. Accordingly, when the toggle valve 33 is opened, this in turn opens the quick-chill control valve 7 to discharge liquid nitrogen through the assembly of spray jets 9. I

Referring now'to FIG. 3, there may be seen another wholly-pneumatic assembly which is quite suitable to function as the control circuit 26 in FIG. 1. In particular, there may be seen a diaphragm-type switching valve 62 which is adapted to be positioned according to the pressure differential measured bythe pitot tube 62A positioned between the fan 17 and the primary cooling coil 14, and the vent probe 628 positioned in the return air flow to the fan l8.When the pressure differential between the pitot tube 62A and the probe 628 increases to a sufficient level because of ice on the primary cooling coil 14, the valve 62 will open to route 17 psig nitrogen pressure from the reducing valve 34 (see FIG. 2) and through a suitable check valve 63 to an accumulator 64 and the actuating port T of a first pneumatic relay 67. The relay 67 is preferably set to open its intake port S to its exit port R whenever the pressure at port T is greater than 7.5 psig (or whatever actuating pressure may be appropriate). Accordingly, pneumatic pressure at 17-20 psig will now pass through ports 5 and R in relay 67 to charge a second accumulator 65 and to apply to the actuating port T of a second relay In the apparatus depicted in FIG. 3, both relays 67 and are preferably adjusted to shift to their R-S condition in response to the same actuating pressure. Ac cordingly, since the actuating pressure being routed to the relay 70 is much greater than 7.5 psig, the relay 70 will now open" to route pressure at l7-20 psig from the reducing valve 34 to the actuator 39 of the two-way valve 38. Accordingly, the valve 38 will now shift to its alternative C-B condition to route pneumatic pressure 30-40 psig or more to actuators 6 and 21.

It will be noted that the pressure in the first accumulator 64 will be blocked by the check valve 63, but will nevertheless bleed away through an orifice 66 which is sized to determine the time of such discharge. Nevertheless, when the pressure in the first accumulator 64 drops below 7.5 psig or whatever actuating pressure may have been chosen for the two relays 67 and 70, the relay 67 will then shift to close its S port and, instead, to open its R port to its vent or B port. It will be noticed that until this occurs, the circuit will have trapped the actuating pressure on port T of the second relay 70.

When the first relay 67 shifts to its R-B condition, however, the 17 psig actuating pressure trapped in the second accumulator 65 will now discharge but only at a rate which is determined by the size of the orifice 68 which is connected in the vent port B of the first relay 67. Nevertheless, when the pressure in the second accumulator 65 drops below 7.5 psig, the second relay will go to its R-B venting condition, and pressure will now be relieved from the actuator 39 through a suitably positioned bleed valve or cock 69. The two-way valve 38 will now revert, or be caused to revert, to its original C-A condition, and the defrost actuating pressure signal 38B will disappear and be replaced by the cooling actuating signal 38A.

Note that the system depicted in FIG. 3 also includes a suitable temperature/pressure regulator 61 with temperature sensor 61A, and that it generates a suitable activating pressure 618 whenever the actual temperature as measured by the sensor 61A is above the temperature which has been set on the regulator 61. The signal 613, of course, is the 17 psig pressure from the reducing valve 34, and thus the shutoff valve 60 must be in its open condition.

Referring again to FIG. I, it will be noted that if the fans 17 and 18 are permitted to continue to circulate air over frozen coils l4 and 15 during the defrosting operation, this will result in pieces of ice and even water being blown off of the coils 14 and I5 and, perhaps, onto the cargo. The temperature within the trailer or other space to be chilled will not necessarily change during the defrost cycle, of course, and thus water being blown off of the coils l4 and 15 may form a coating of ice over the cargo.

Referring now to FIG. 4, there may be seen a pictorial representation of one embodiment of apparatus which eliminatesthis disadvantage by de-activating the pneumatic motor 16 during the defrost cycle. In particular, there may be seen a suitable coil assembly 80 having a primary winding 78 and a secondary winding 79, and further being coupled to receive air flow from a blower 89 which is actuated by a pneumatic motor 88. A conduit 77 is interconnected to route 40 psig nitrogen pressure from the cooling control valve 3 and check valve It (see FIG. 1) to the input ports 76A of the primary winding 78, and another conduit 82 is coupled to route nitrogen vapor or pressure from the exit ports 78B of the primary winding 78 to the AB port of a three-way valve 83.

The A port of the valve 83 is preferably coupled through a conduit 85 to the intake port of the pneumatic motor 88. The B port of the valve 83 is not only cou pled to the exit port of the motor 88; it is also preferably coupled to the intake port 79A of the secondary winding 79 of the coil assembly 80.

The three-way valve 83 may be seen to be positioned by a two-position actuator 84 of conventional design which is coupled to the control circuit 26 which is depicted in FIG. 1 and which may therefore be actuated by the circuitry depicted in either FIG. 2 or FIG. 3. More particularly, the first conduit 84A will preferably be coupled to receive the so-called cooling pneumatic signal 26A, whereas the other conduit 84B functions to couple the defrost pneumatic signal 268 to the actuator 84. Accordingly, when the cooling signal or pressure 26A is applied to the actuator 84, the valve 83 will be moved to its AB-A position to route nitrogen pressure from conduits 82 and 85 through the pneumatic motor 88 and into the exit conduit 86. The other port B in the valve 83 is closed, and thus the pressure exiting the motor 88 is routed through the conduit 87 to the intake port 7 9A of the secondary winding 79. After traversing the secondary winding 79, the nitrogen vapor will be discharged by way of the conduit 81 which is coupled to its discharge port 793.

When the control circuit 26 is caused to replace the cooling signal 26A with the defrost signal 268, however, this results in pressure being removed from the conduit 84A and being applied instead to conduit 848. The actuator 84 will now shift the three-way valve 83 to its alternative AB-B condition, blocking port A and opening port B into conduits 86 and 87. The cooling control valve 3 will now be closed, but heated nitrogen vapor will now flow through the other check valve 11 and conduit 76 to the primary winding 78. When the hot nitrogen vapor exits the primary winding 78, however, and passes through the conduit 82 to the AB port of the three-way valve 83, it is routed through port B to the two conduits 86 and 87.

Nitrogen pressure cannot drive the pneumatic motor 88 in a reverse direction, since port A of the valve 33 is now blocked. Thus, hot nitrogen pressure will bypass the motor 88 and pass directly through the conduit 87 to the secondary winding 79 of the coil assembly 80.

Although the only gas which has been referred to herein for motivating the system has been nitrogen, it should be clearly understood that many different gases can be liquefied and can be employed for purposes of the present invention. Nevertheless, the inert character of nitrogen as well as its abundance and relatively low cost makes it especially useful.

Also, the fact that the present invention is especially suitable for use in refrigerating large over-the-road trailers should not be construed as an indication that this is the only use for these methods and apparatus. On

the contrary, the present invention may be used in many applications, such as maritime vessels and aircraft, and for large stationary establishments wherein attendance is either impractical or undesirable.

As hereinbefore stated, it is a particular feature of the present invention to provide means and methods for maintaining a preselected ambient temperature within the trailer or other space to be refrigerated, instead of merely cooling such space below a preselected maximum temperature, as is the case with many of the systems of the prior art. In other words, it is a feature of the system hereinbefore depicted and described that the subject system includes means by which the primary and secondary coils 141 and 15 may be caused to heat the air being driven over them by the blowers 17 and 18, as well as to cool such air.

Referring again to FIG. 1, it will be noted that the temperature/pressure regulator 27 may be of the type of regulator 3H depicted in FIG. 2, which is adapted to either open or close, depending on whether the temperature sensed by the sensor 31A is either above or below the setting of the regulator 31. In such a case, the regulator 31 will only produce a pressure signal 313 when the temperature measured by the sensor 31A is above the setting on the regulator 31, and if the measured temperature is equal to or less than the setting on the regulator 31, there will be no pressure signal 318 whatsoever. Thus, the cooling control valve 3 will merely close, and nitrogen flow through the coils 14 and 15 will stop. Nor will any nitrogen pass into and through the heat exchange coil 12, since the control circuit 26 which is depicted in FIG. 2 will not produce the defrost signal 26B merely because the cooling signal 26A is cancelled.

Referring to FIG. 3, however, it should be mentioned that the temperature/pressure regulator 6R, which is a part of this control circuit 26, is preferably of the type which generates a variable pressure signal 61 which is functionally related in magnitude to the temperature differential between the actual ambient temperature and the temperature sought to be established and 'maintained. More particularly, if the actual ambient temperature as indicated by the sensor 61A is greater than the temperature setting on the regulator 6i, the regular 61 will produce a signal pressure 618 which .is of some preselected maximum such as 17 psig, for example. However, if the ambient temperature in the trailer or other space being refrigerated drops to a level which is either equal to or not more than a preselected amount (such as 4 F, for example) below the setting on the regulator 61, the regulator 61 will not abruptly and totally discontinue the pressure signal 618; it will merely reduce the magnitude of such pressure accordingly. The actuator 41, however, is preferably selected to respond only to an actuating pressure greater than a preselected minimum such as 14 psig, for example. The regulator 61, of course, begins to reduce the magnitude of the signal pressure 618 to below 17 psig as the ambient temperature nears the setting of the regulator 61, and thus the pressure of the signal 613 on the actuator 41 preferably drops below the 14 psig'value to cause the valve 40 to discontinue its output signal pressure 40A just as the ambient temperature indicated by the sensor 61A drops to the value of the setting on the regulator 61.

It will thus be apparent that the component hereinbefore referred to as a valve" 40 is preferably a combination relay and valve, which may be of any of several conventional designs, and which is preferably arranged to discontinue its regular output pressure 40A to port C of the valve 38 whenever the magnitude of the pressure signal 618 drops below 14 psig of whatever upper actuating pressure may have been selected. As hereinbefore stated, the magnitude of the pressure signal 613 is a function of the difference between the actual temperature and the temperature setting on the regulator 61. Thus, if the actual temperature continues to decline, the magnitude of the pressure signal 618 being generated or transmitted by the regulator 61 will continue to decline proportionately, although not necessarily equally. The system depicted in H6. 3 is preferably made nonresponsive to small fluctuations of the actual temperature, nevertheless, or else the system might cycle back and forth in a manner which might be damaging to the system depicted in FIG. 1.

It is preferable that the control system depicted in FIG. 3 react to any significant decline of the actual temperature below that set on the regulator 61, however, such as to a decline of 4 F. or greater. Furthermore, the regulator 61 is preferably adapted to reduce the magnitude of the pressure signal 61B as the actual temperature decreases, as hereinbefore stated. Accordingly, the regulator 611 may conveniently be set to reduce the pressure signal MB to 3 psig or less (for example) as the actual temperature drops 4 F. or more below whatever the setting may be on the regulator 61. In addition, the relay/valve 39 is preferably adjusted to respond to this reduction in pressure of the signal 618 to open and route 50 psig nitrogen pressure through in the form of signal 408 to the upstream side of a downwardly-opening check valve $0, and also into conduit 84A which is illustrated in H6. 4.

It will be noted that the 5b psig nitrogen pressure 40B which traverses the check valve 90 is routed by way of the defrost signal 388 to the two actuators 6 and 2K, and this will open the defrost control valve 5, the control valve 20, and the fuel control valve 22, whereupon butane or the like will now flow from the fuel bottle 24 through the fuel shutoff valve 23, to the burner 25, to be ignited by the pilot flame 56. This 55 psig nitrogen pressure 408 will also be routed to conduit 848. Since the same amount of pressure is simultaneously being routed into the other conduit 841A, however, the actuator 84 will continue to maintain its AB-A condition. Thus, heated nitrogen vapor will flow from the primary coil 80 through the pneumatic motor d8, whenever the actual temperature as measured by the sensor 63A is 4 F. (or whatever value is selected) or more below the setting on the regulator 61. Consequently, the blower 89 will now be activated instead of being inactivated, as in the case of the defrost mode," and warmed air will now be circulated into and through the space in the trailer.

it will be apparent from the foregoing that many other variations and modifications may be made in the structures and methods described herein without substantially departing from the basic concept of the present invention. Accordingly, it should be clearly understood that the forms of the invention described herein and depicted in the accompanying drawing are exemplary only and are not intended as limitations in the scope of the present invention.

What is claimed is:

i. A temperature control system for establishing and maintaining the ambient temperature within a sealed enclosure, comprising a pressure vessel for storing a quantity of liquified gas under a pressure,

a first hollow thermal exchange means disposed in said enclosure and having an intake end for receiving said gas in liquid form and an exit end for discharging said gas in vapor form,

a second hollow thermal exchange means also disposed in said enclosure and having an intake end for receiving said gas in vapor form and an exit end for venting said gas therefrom,

blower means disposed in said enclosure in functional relationship to said first thermal exchange means,

a pneumatically-actuated motor arranged for driving said blower means and actuated by the flow of said gas vapor from said first thermal exchange means to said second thermal exchange means,

sensing means responsive to the accumulation of ice on the exterior of at least one of said thermal exchange means, and

heating means responsive to said sensing means for heating gas received by said first thermal exchange means from said pressure vessei.

2. The system described in claim 1, further including switching means interconnected therewith for inactivating said motor upon actuation of said heating means.

3. A temperature control system for establishing and maintaining the ambient temperature within a generally confined space, comprising a pressure vessel for storing a quantity of liquefied gas under a pressure,

a primary heat exchanger means disposed in said space and having an intake end for receiving a flow of said gas from said vessel and an exit end for discharging said received gas in vapor form,

a secondary heat exchanger means disposed in said space and having an intake end for receiving said gas in vapor form and an exit end for venting said received gas,

restriction means for interconnecting said exit end of said primary heat exchanger to said intake end of said secondary heat exchanger,

heating means for receiving and vaporizing liquefied gas from said vessel and for heating said received vaporized gas to a preselected temperature,

defrosting means for routing gas flow from said vessel through said heating means to said primary heat exchanger means upon the accumulation of ice by one of said heat exchangers.

d. The system described in claim 3, wherein said switching means further comprises control means interconnected with said heating means and said heat exchanger means for bypassing said heating means and for routing liquefied gas from said vessel to said primary heat exchanger means upon the occurrence of an ambient temperature greater than a preselected magnitude.

5. The control means described inclaim a, further including first sensor means functionally responsive to the occurrence of an ambient temperature greater than a first preselected magnitude, and

second sensor means functionally responsive to the occurrence of an ambient temperature less than a second preselected magnitude.

6. The control means described in claim 5, wherein said restriction means includes blower means motivated by vapor flow between said primary and secondary heat exchanger means for circulating the atmosphere in said space across said heat exchanger means, and

wherein said system further includes disconnect means for bypassing said blower means and routing vapor flow directly from said primary means to said secondary means during actuation of said heating means.

7. The control means described in claim 6, wherein said second sensor means is responsive to a function of the pressure of said atmosphere being circulated across said one of said heat exchanger means.

8. The system described in claim 7, wherein said control means further includes a temperature controller responsive to said first sensor means for comparing the actual ambient temperature in said space with the preselected temperature sought to be established and maintained therein.

9. The system described in claim 8, wherein said controller is interconnected therewith for routing liquefied gas from said vessel substantially directly to said primary heat exchanger means when said actual ambient temperature is greater than said preselected temperature, and

wherein said controller routes said liquefied gas from said vessel to said primary heat exchanger means by way of said heating means when said actual ambient temperature is less than said preselected temperature sought to be established and maintained in said space.

10. A refrigeration system for a movable vehicle and the like, comprising a pressure vessel for containing a supply of liquified gas,

a first heat exchanger coil disposed in said vehicle and having intake and outlet ends,

a second heat exchanger coil disposed in said vehicle and also having intake and outlet means,

blower means for routing air across said coils and actuated by gas flow from said first coil into said second coil,

defrosting means responsive to accumulation of frost and the like on said coils for heating gas flowing into said first coil from said vessel.

11. The system described in claim 10, wherein said defrosting means includes detector means adjacent said coils for indicating accumulation of frost or the like thereon, and

heating means responsive to said detector means for heating said gas flowing into said first coil from said vessel.

12. The system described in claim 11, wherein said detector means is responsive to a change in air pressure across said coils.

13. The system described in claim 12, wherein said defrosting means further includes means for deactuating said blower means in response to said detector means.

14. The system described in claim 13, further including temperature sensing means responsive to the temperature of the air routed across said coils by said blower means, and control means for interrupting gas flow into said first coil in resonse to said temperature sensing means. 15. A refrigeration system for a movable vehicle and the like, comprising a pressure vessel for containing a supply of liquified gas,

a first hollow thermal exchange means disposed within said vehicle and having an intake end for receiving said gas from said vessel and an exit end for discharging said gas in vapor form,

a second hollow thermal exchange means also disposed within said vehicle and having an intake end for receiving said gas in vapor form and an exit end for discharging said received gas from said vehicle,

a pneumatically-energized blower for routing air across said exchange means in response to vapor flow from said first exchange means to said second exchange means,

heating means for heating said gas flowing from said pressure vessel to said first and second thermal exchange means, and

control means for routing gas from said vessel into said first exchange means in response to a temperature in said vehicle greater than a preselected magnitude and for interrupting gas from said vessel into said first exchange means in response to a temperature in said vehicle less than said preselected magnitude,

said control means, being further responsive to the accumulation of frozen atmospheric condensate on said exchange means for actuating said heating means.

16. The system described in claim 15, wherein said control means is further interconnected to inactivate said blower upon actuation of said heating means. 

1. A temperature control system for establishing and maintaining the ambient temperature within a sealed enclosure, comprising a pressure vessel for storing a quantity of liquified gas under a pressure, a first hollow thermal exchange means disposed in said enclosure and having an intake end for receiving said gas in liquid form and an exit end for discharging said gas in vapor form, a second hollow thermal exchange means also disposed in said enclosure and having an intake end for receiving said gas in vapor form and an exit end for venting said gas therefrom, blower means disposed in said enclosure in functional relationship to said first thermal exchange means, a pneumatically-actuated motor arranged for driving said blower means and actuated by the flow of said gas vapor from said first thermal exchange means to said second thermal exchange means, sensing means responsive to the accumulation of ice on the exterior of at least one of said thermal exchange means, and heating means responsive to said sensing means for heating gas received by said first thermal exchange means from said pressure vessel.
 2. The system described in claim 1, further including switching means interconnected therewith for inactivating said motor upon actuation of said heating means.
 3. A temperature control system for establishing and maintaining the ambient temperature within a generally confined space, comprising a pressure vessel for storing a quantity of liquefied gas under a pressure, a primary heat exchanger means disposed in said space and having an intake end for receiving a flow of said gas from said vessel and an exit end for discharging said received gas in vapor form, a secondary heat exchanger means disposed in Said space and having an intake end for receiving said gas in vapor form and an exit end for venting said received gas, restriction means for interconnecting said exit end of said primary heat exchanger to said intake end of said secondary heat exchanger, heating means for receiving and vaporizing liquefied gas from said vessel and for heating said received vaporized gas to a preselected temperature, defrosting means for routing gas flow from said vessel through said heating means to said primary heat exchanger means upon the accumulation of ice by one of said heat exchangers.
 4. The system described in claim 3, wherein said switching means further comprises control means interconnected with said heating means and said heat exchanger means for bypassing said heating means and for routing liquefied gas from said vessel to said primary heat exchanger means upon the occurrence of an ambient temperature greater than a preselected magnitude.
 5. The control means described in claim 4, further including first sensor means functionally responsive to the occurrence of an ambient temperature greater than a first preselected magnitude, and second sensor means functionally responsive to the occurrence of an ambient temperature less than a second preselected magnitude.
 6. The control means described in claim 5, wherein said restriction means includes blower means motivated by vapor flow between said primary and secondary heat exchanger means for circulating the atmosphere in said space across said heat exchanger means, and wherein said system further includes disconnect means for bypassing said blower means and routing vapor flow directly from said primary means to said secondary means during actuation of said heating means.
 7. The control means described in claim 6, wherein said second sensor means is responsive to a function of the pressure of said atmosphere being circulated across said one of said heat exchanger means.
 8. The system described in claim 7, wherein said control means further includes a temperature controller responsive to said first sensor means for comparing the actual ambient temperature in said space with the preselected temperature sought to be established and maintained therein.
 9. The system described in claim 8, wherein said controller is interconnected therewith for routing liquefied gas from said vessel substantially directly to said primary heat exchanger means when said actual ambient temperature is greater than said preselected temperature, and wherein said controller routes said liquefied gas from said vessel to said primary heat exchanger means by way of said heating means when said actual ambient temperature is less than said preselected temperature sought to be established and maintained in said space.
 10. A refrigeration system for a movable vehicle and the like, comprising a pressure vessel for containing a supply of liquified gas, a first heat exchanger coil disposed in said vehicle and having intake and outlet ends, a second heat exchanger coil disposed in said vehicle and also having intake and outlet means, blower means for routing air across said coils and actuated by gas flow from said first coil into said second coil, defrosting means responsive to accumulation of frost and the like on said coils for heating gas flowing into said first coil from said vessel.
 11. The system described in claim 10, wherein said defrosting means includes detector means adjacent said coils for indicating accumulation of frost or the like thereon, and heating means responsive to said detector means for heating said gas flowing into said first coil from said vessel.
 12. The system described in claim 11, wherein said detector means is responsive to a change in air pressure across said coils.
 13. The system described in claim 12, wherein said defrosting means further includes means for deactuating said blower means in response to said detector means.
 14. The system described in claim 13, further including temperature sensing means responsive to the temperature of the air routed across said coils by said blower means, and control means for interrupting gas flow into said first coil in resonse to said temperature sensing means.
 15. A refrigeration system for a movable vehicle and the like, comprising a pressure vessel for containing a supply of liquified gas, a first hollow thermal exchange means disposed within said vehicle and having an intake end for receiving said gas from said vessel and an exit end for discharging said gas in vapor form, a second hollow thermal exchange means also disposed within said vehicle and having an intake end for receiving said gas in vapor form and an exit end for discharging said received gas from said vehicle, a pneumatically-energized blower for routing air across said exchange means in response to vapor flow from said first exchange means to said second exchange means, heating means for heating said gas flowing from said pressure vessel to said first and second thermal exchange means, and control means for routing gas from said vessel into said first exchange means in response to a temperature in said vehicle greater than a preselected magnitude and for interrupting gas from said vessel into said first exchange means in response to a temperature in said vehicle less than said preselected magnitude, said control means being further responsive to the accumulation of frozen atmospheric condensate on said exchange means for actuating said heating means.
 16. The system described in claim 15, wherein said control means is further interconnected to inactivate said blower upon actuation of said heating means. 